Wave energy has a large potential to become an interesting and important cost efficient source of electrical power due to the high energy density of waves in the ocean. Furthermore, it is believed that wave energy is more predictable and more consistently available than wind power. The wave energy is captured by means of a wave energy converter (WEC).
A wave energy converter as used in this application is defined as a system for converting wave energy into electrical energy.
A number of different types of wave energy converters exist which are categorized based on their size, required water depth, working principle, . . . . Based on the method used to capture the energy of the waves, one may for instance distinguish between point absorbers or buoys, attenuators, terminator devices, oscillating water column devices and overtopping devices.
A wave energy converter comprises an element which is movable under the influence of the wave motion. A wave-induced movement of the buoy, or of the movable element in general, typically is a bidirectional movement or oscillatory movement or pendular movement, causing a mechanical shaft connected to the movable element to rotate alternatively in positive and negative direction. This mechanical shaft is then further connected to the rotor of an electrical machine operating as an electric generator. As such the mechanical energy can be transformed into electrical energy. The part of the WEC being responsible for transforming absorbed wave energy into electrical energy is typically referred to as the power-take-off (PTO) system of a WEC.
Efficiently transforming the energy comprised in the pendular or oscillatory movements of the movable element into electrical energy is hard. Indeed, such oscillatory movements cause the rotor of the electric generator to undergo a bidirectional rotational movement, i.e. to consecutively come to stand-still, to accelerate to a maximum rotation speed in a first direction, to decelerate and to come to stand-still once more, to accelerate to a maximum rotation speed in a second direction, opposite the first direction, to decelerate and to come to stand-still once more. As a consequence the control of such a machine becomes quite difficult on the one hand and on the other hand the electrical machine operates in conditions of torque and speed which are far off the nominal and optimal operating points.
Therefore, there have been several attempts to transform the oscillatory movement of the movable element in a unidirectional movement of the rotor of the generator. Such attempts include the use of special types of gearboxes such as planetary gearboxes as presented in e.g. WO2011/126451, WO2006/118482, WO2011/092555. All these examples use at least one, typically two clutches, freewheels or other anti-reverse mechanisms. Such clutches, freewheels and anti-reverse mechanisms are then continuously engaged and disengaged to ensure the wave-induced oscillatory movement is converted and inverted to realize a unidirectional movement of the rotor of the generator. However, the continuous engagement and disengagement of such freewheels or anti-reverse mechanisms cause mechanical losses in the PTO system as well does it cause the wave energy converter's PTO system to be prone to mechanical wear reducing its lifetime. Therefore, there is still a need for more efficient and/or more robust PTO systems.
In “The Electric Variable Transmission”, IEEE Transactions on Industry Applications, Vol 42, No 4, pp. 1092-1100, July/August 2006, Martin J. Hoeijmakers and Jan A. Ferreira describe an electromechanical device referred to as “EVT”. This publication is incorporated herein by reference in its entirety, and will be referred to herein as [Hoeijmakers]. The basic principles of the EVT will be briefly described below, in relation to FIG. 17 to FIG. 19, which are a replica of FIG. 2, FIG. 3 and FIG. 6 of said IEEE-publication.